In recent years, multiband compatible wireless communication terminals which can utilize a plurality of frequency bands have been developed. In these multiband compatible wireless communication terminals and wireless communication systems utilizing a plurality of frequency bands, when the wireless communication terminal moves, the terminal becomes necessary to switch from a base station communicated with at present to a base station using a different frequency band.
The function of switching base stations (communication channels) communicated with is called “hand-off” (or “hand-over”). Hand-off includes two types: soft hand-off and hard hand-off (see, for example, Patent Document 1).
“Soft hand-off” is hand-off performed without switching frequencies. The base station communicated with at present (handing-off channel) and the base station desired to be newly communicated with (handed-off channel) are temporarily set in a state of simultaneous communication, then the processing for switching the base stations is carried out.
As described above, in soft hand-off, the wireless communication terminal is always communicating with one or more base stations, therefore, communication is not interrupted at the time of hand-off.
However, soft hand-off is not always possible. Soft hand-off cannot be carried out in a case where two base stations cannot provide services to that wireless communication terminal (mobile terminal) by the same frequency, therefore hard hand-off is performed.
“Hard hand-off” maintains communication with a first base station up to just before switching communication to a second base station when changing from the first base station to the second base station, then switches the frequency band. For this reason, the channel in use ends up being temporarily released, so communication ends up being interrupted transiently when switching frequency bands.    Patent Document 1: Japanese Patent Publication (A) No. 2003-244742